Small objects including liquids can be trapped and manipulated without contact by an acoustic standing wave, and this technique is applicable to noncontact transportation and measurements of the physical properties of liquids. This paper discusses the internal flow of a resonating droplet levitated in an acoustic standing wave. The experimental system consists of a vibrating disc with four bolt-clamped Langevin-type transducers and a circular reflector, resulting in an acoustic standing wave between them. A 10-μL water droplet containing microparticles was injected near the pressure node. The vibration of the droplet was induced by applying AM signals, and its behavior was observed using a high-speed camera. The internal flow was analyzed using particle image velocimetry. The findings indicated that the maximum internal velocity was 0.23 m/s for the second mode, while the velocity decreased to 0.0016 m/s for the fourth mode, which is approximately one-one hundred and fiftyth of the velocity in the second mode. This difference can be attributed to the internal vortex flow generated by the surface wave of the droplet in the fourth mode. These findings imply that the mass transport and mixing efficiency within a droplet can be controlled by selectively exciting a specific vibration mode.
Hirayama et al. (2025) studied this question.